1. Field of the Invention
The present invention generally relates to implantable medical devices, and more particularly, relates to electrodes for electrically coupling to nerve tissue.
2. Description of the Prior Art
The use of electrodes to monitor electrical activity and stimulate body tissue is quite old. U.S. Pat. No. 1,662,446 issued to R. H. Wappler teaches an early electrode system. The Wappler electrode is used for acute stimulation only, and is not implantable.
An early stimulation electrode which is chronically implantable is taught by S. I. Schwartz, et al. in U.S. Pat. No. 3,421,511, herein incorporated by reference. U.S. Pat. No. 3,654,933 issued to Hagfors, herein incorporated by reference, teaches an improved stimulation electrode for chronic implantation. Clinical experience with the electrodes taught by Schwartz et al. and Hagfors has shown a potential for excess irritation in certain applications. This irritation results in swelling of the nerve tissue which is exacerbated by the fixed diameter of the cuff of each device after being sutured into place.
U.S. Pat. No. 3,738,368 issued to Avery, et al., U.S. Pat. No. 3,774,618 issued to Avery, and U.S. Pat. No. 3,955,560 issued to Stein, et al. all teach electrodes having fixed diameters after chronic implantation. These electrodes also tend to cover relatively large areas of the nerve tissue, thus depriving nutrient access to great numbers of adjacent nerve cells.
U.S. Pat. No. 4,573,481 issued to Bullara suggests a neural electrode array having a serpentine shape. This shape provides significant access to nutrients by the nerve. This structure, however, provides difficulty in implantation. Because an increase in diameter of the electrode system requires a decrease in length, it is anticipated that the Bullara electrode system does not provide a satisfactory solution to the problem of nerve swelling.